It is known that particles in a sample can be characterised by illuminating the sample and measuring the light scattered by the particles. The particles of the sample are typically dispersed within a sample cell in a dispersant medium during measuring. The dispersant medium is typically air or water, and typically flows through the sample cell during measurement.
The correlation between light scattering and particle characteristics can be described by the well-known Mie solution to Maxwell's equations. Smaller particles tend to result in larger scattering angles, and larger particles result in smaller scattering angles. The light scattered at each of a range of angles from the sample can be used to determine, for example, a size distribution of the particles in the sample. Such a measurement may be referred to as a light (e.g. laser) diffraction particle characterisation.
Much of the development of instruments for light diffraction particle characterisation has been directed towards increasing the size range of particles that can be characterised at one time. At the same time, there is a demand to reduce the size of the instrument. The requirements for a greater measurement range and a smaller instrument are in conflict, which may result in technical difficulties in achieving sufficient performance, or a reduced quality of measurement. In particular, the technical requirements for achieving accurate characterisation of larger particles are particularly challenging and expensive. There may be a discrepancy between the cost of the components needed to achieve large particle characterisation and the perceived value associated with these measurements.
An instrument for characterisation of particles by light diffraction typically works by measuring the intensity of light scattered by fine particles suspended in a strong monochromatic light source of known intensity. The instrument needs to measure the intensity of light at a series of angles measured from the illumination direction, because different sizes of particles scatter light at different angles. Generally a large particle will scatter light at an angle very close to the axis of the illuminating beam, and a smaller particle will scatter light at a larger angle. Because the illuminating beam is much stronger than the scattered light a detector is typically used that allows light to pass through without touching the detector. Otherwise, the illumination beam incident on the detector would produce a very large reflection that can leak into neighbouring detectors. The reflected light would tend to bounce all around the inside of the instrument, overwhelming the much smaller scattered light signals.
Larger particles scatter light at angles close to the axis of the illumination beam (e.g. a laser). To separate the scattered light from the illumination light it is necessary either to measure with detectors close to the focused spot of the illumination beam, or to use a longer focal length in order to allow the illumination beam and scattered light to separate out. The former approach means that the detector and illumination beam must be very accurately aligned, and the second approach results in a very long instrument that may have stability problems.
As particle size becomes smaller, the useful scattered light changes in two ways. The peak intensity is scattered at a larger angle to the illumination beam axis and the scattering becomes more isotropic. A size of particle will eventually be reached where the scattered light is almost completely isotropic, and therefore the instrument cannot tell the difference between particles at this size and particles that are smaller. This sets the lower size limit for a light diffraction instrument. However, because the particle size at which the scattering becomes isotropic depends on the wavelength of the light, it is possible to extend the bottom size limit for an instrument by changing to a shorter wavelength light source.
One approach is to use a red Helium Neon laser at 633 nm to measure the largest particles and a blue non-coherent light source (such as an LED or filtered incandescent lamp) to allow measurement of the fine particles. A large component of the cost of such an instrument is associated with the components needed to measure the large particles: the laser must have a very high beam quality, and the detector must be positioned to almost sub-micron tolerances.
One solution might be to limit the maximum particle size that may be characterised by the instrument, but even when measuring fine particles it is often desirable to be confident that there are no large particles present. Any system that limits the top size too far may also limit the ability to report problems with aggregates and contaminants.
A solution that addresses or ameliorates at least some of the above mentioned problems is desired.